1. Field of the Invention
The present invention relates to a seal structure of a fuel cell of a polymer electrolyte-type.
2. Description of Related Art
A PEFC (Polymer Electrolyte Fuel Cell) apparatus includes individual fuel cells. Each fuel cell includes a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane and a pair of electrodes disposed on opposite sides of the electrolyte membrane. The pair of electrodes includes an anode provided on one side of the membrane and constructed of a first catalyst layer and a cathode provided on the other side of the membrane and constructed of a second catalyst layer. A first diffusion layer may be provided between the first catalyst layer and a first separator and a second diffusion layer may be provided between the second catalyst layer and a second separator. The first separator has a passage formed therein for supplying fuel gas (hydrogen) to the anode and the second separator has a passage formed therein for oxidant gas (oxygen, usually, air) to the cathode. At least one layer of the fuel cell 1 constructs a module. A number of modules are piled, and electrical terminals, electrical insulators, and end plates are disposed at opposite ends of the pile of modules to construct a stack of fuel cells. After tightening the stack of fuel cells between the opposite end plates in a fuel cell stacking direction, the end plates are coupled to a fastening member (for example, a tension plate) extending in a fuel cell stacking direction outside the pile of fuel cells by bolts extending perpendicularly to the fuel cell stacking direction.
In the PEFC, at the anode, hydrogen is changed to positively charged hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte membrane to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator) to form water as follows:At the anode: H2→2H++2e−At the cathode: 2H++2e−+(½)O2→H2O
In order that the above reaction is conducted, fuel gas and oxidant gas are supplied to the stack. Further, since the fuel cell temperature rises due to the heat generated at the water production reaction and a Joulean heat, a coolant passage is formed at every cell or at every module, and a coolant (usually, cooling water) is caused to flow in the coolant passage. In order to prevent the fuel gas, the oxidant gas, and the coolant from leaking from respective passages, every fuel cell is sealed between the separators thereof.
Japanese Patent Publication No. HEI 11-154522 discloses use of fluid sealant (liquid gasket) to seal between the separators.
However, with the conventional seal structure of a fuel cell, there are the following problems:
First, it is difficult to assure a uniform seal. Since a thickness of the sealant differs by a thickness of the electrolyte membrane between at a region where an electrolyte membrane exists and at another region where the electrolyte membrane does not exist, a seal pressure imposed on the sealant changes and a uniform seal pressure is not assured. More particularly, at the region where the electrolyte membrane exists a large pressure acts on the sealant and the creep of the sealant is large so that the seal pressure decreases with the lapse of time, which is accompanied by loosening of the tightening force of the stack and thereby decreasing the output the fuel cell.
Second, not only is there the above seal pressure variance due to whether the electrolyte membrane exists or not between the seal surfaces, but there also is a variance in the coated thickness of the sealant which makes it difficult to obtain a uniform seal pressure. At a place where an over-pressure acts on the sealant, the sealant may be bulged out from between the seal surfaces. If the bulged sealant decreases a cross-sectional area of a gas passage or blocks the gas passage, the output of the fuel cell will be badly affected.